The present invention relates in general to switching mode driving techniques of an inductive load, and in particular, to the control of the current circulating in the inductive load.
There is an increasing use of digital systems not only for driving motors or electromagnetic actuators, but also for analog signal processing applications, such as, for example, in an audio amplifier for reducing power consumption. This implies the use of final driver stages for driving inductive loads, wherein the driver stages function in a switching mode, typically, but not exclusively, according to a pulse width modulation (PWM) technique and variations thereof. Driving an inductive load in such a mode brings about the presence of a certain ripple, which has a limited and tolerable amplitude, on the output waveforms. In these applications, it is necessary to precisely control the current circulating in the external inductive load. This is commonly implemented by a feedback loop that regulates the relative duration of alternating magnetization and demagnetization phases.
The ability of PWM driving circuits to track the dynamics (voltage swing) of an analog input signal even during abrupt amplitude changes, and the requirement of minimizing the residual ripple on the output waveform imposes contrasting electrical requirements on the discharge current path of the load inductance during the demagnetization phases that alternate to the magnetization phases at the base or mean switching frequency.
Independent of the type of application, the base switching frequency, which is fixed for a constant frequency mode, or the interval of frequencies between a minimum and maximum switching frequency according to the variable frequency mode are established above the audible band that extends up to about 20 KHz. This is done to avoid generation of perceivable disturbances. Considering that the losses in the final stage due to the switching of the power switches increase with the switching frequency, it is advantageous to design such a fixed switching frequency or fixed mean frequency value in a range that is commonly between 20 and 30 KHz.
It is also known to effect the demagnetization phase according to a first mode commonly known as slow (current) recirculation, or according to a second mode commonly known as fast (current) recirculation. In the first mode of discharging the load inductance, the inductive load is normally short circuited so that the current may quickly decay as a function of only the electrical constant of the load itself. In the second mode of discharge, the load is applied a voltage of an opposite sign of the voltage that was applied thereto during the preceding magnetization phase for speeding-up (forcing) the discharge of the inductance.
It is known that by determining an automatic selection of either the first or the second discharge mode, or a combination of the two, the residual ripple is reduced and at the same time the speed for closely tracking the analog signal is preserved. Typically, a current mode driving of the load is implemented by monitoring on a sensing resistor the current flowing through the load, and using the voltage drop on the current sensing resistor as a feedback control signal.
The control logic of the final power stage detects any eventual shift of the actual current value from the desired value, and intervenes to modify the driving for bringing back the value of the current to the desired level. Numerous techniques are known for optimizing the performance by asserting a choice between a slow recirculation demagnetization and a fast recirculation demagnetization, or a mix of the two modes, as a function of certain parameters that are monitored during operation.
U.S. Pat. No. 4,743,824 discloses a driving method according to which, within each switching period, a magnetization phase and a demagnetization phase are performed. The latter may be conducted according to a slow recirculation mode or a fast recirculation mode. The slow recirculation mode is selected when the reference current value at the beginning of the switching period exceeds the value that it had at the beginning of the preceding switching period.
In contrast, the fast recirculation mode is selected when the reference current value at the start of the switching period is less than the value it had at the start of the preceding switching period. This method has the drawback of not accounting for the real current flowing in the load, and in case the reference signal is periodic, may generate spurious spectral components of relatively low frequency.
The commercial device having code number 3955, of Allegro Microsystems Incorporated, performs within each switching period a magnetization and a demagnetization phase. The latter is composed of a first part or sub-phase during which a fast recirculation demagnetization takes place followed by a second part or sub-phase during which a slow recirculation demagnetization is carried out. The ratio between the duration of the two parts of the demagnetization phase is established by an analog signal provided at the input of the device.
The method implemented in the above noted commercial device has the drawback of not accounting for the actual current flowing in the load unless complex ancillary circuitry is specifically formed. This is represented by the fact that within every switching period for a demagnetization phase, the switching losses in the power switches are relatively high. This is because a sub-phase is carried out according to a fast recirculation mode.
U.S. Pat. No. 6,119,046 discloses a method according to which, within each switching period, a phase of magnetization and a phase of demagnetization are contemplated. The demagnetization phase is alternatively conducted according to a slow recirculation mode or according to a fast recirculation mode. It may also comprise a first part conducted according to a fast recirculation mode followed by a second part conducted according to a slow recirculation mode. The selection of the mode with which the demagnetization phase begins is made at the start of each switching period. This is done as a function of the difference between the reference value and the actual current value in the load. This method has the drawback of requiring that such a difference be the same and stored until the instant in which the demagnetization phase starts.
According to another method described in European Patent No. 613,235, each switching period may comprise only a magnetization phase, only a demagnetization phase, or a phase of magnetization and a phase of demagnetization. The phase of demagnetization may be conducted according to a slow recirculation mode or a fast recirculation mode. The duration of the magnetization and demagnetization, as well as the mode with which the demagnetization phase is eventually performed, are established at the start of each switching period. This is done as a function of the value of the reference current or, alternatively, of the value of the current flowing in the load and of the difference between the reference value and the value of the current flowing in the load. This method has the drawback of requiring that the mode of carrying out the eventual demagnetization phase, which, as said above, must be selected at the start of each switching period, be stored until the instant in which the demagnetization phase starts.
According to another method disclosed in U.S. Pat. No. 4,904,562, a magnetization phase may precede or follow only a demagnetization phase conducted according to a slow recirculation mode, and a demagnetization phase conducted according to a fast recirculation mode may precede or follow only a phase of demagnetization conducted according to a slow recirculation mode. This method permits the amplitude of the residual ripple to be reduced, but does not improve performance as far as the speed of response is concerned.
In view of the foregoing background, an object of the present invention is to provide a method for driving in a switching mode an inductive load that eliminates or overcomes the above noted drawbacks and limitations of the known techniques.
According to the method of the invention, two phases that are selected among a magnetization phase, a slow recirculation demagnetization phase and a fast recirculation demagnetization phase, are carried out during each switching period or cycle. The two phases to be carried out are chosen as a function of the instantaneous conditions of operation that are constantly monitored in either a continuous mode or a sampled mode.
In particular, selection between a slow recirculation demagnetization phase and a fast recirculation demagnetization phase during a whole switching cycle is a function of the result of the comparison of a signal representative of the instantaneous value of the current flowing in the load with a pair of upper and lower threshold values.
According to a first preferred embodiment of the invention, each switching period begins with a magnetization phase to which a fast recirculation demagnetization phase follows whenever the signal representative of the current in the load exceeds the upper comparison threshold. If the instantaneous value of this signal drops below the lower comparison threshold before the next switching cycle begins, a slow recirculation demagnetization phase is performed.
The driving modulation (for example, a PWM type) may be performed with either a switching period of a pre-established duration (that is, according to a constant frequency mode) or the duration of the period may be variable (that is, according to a variable frequency mode), for example, by maintaining constant the duration of the demagnetization phase within each switching period of variable duration.
According to another embodiment of the invention, which is particularly suited in a constant frequency modulation, the signal representative of the current flowing in the load is compared at the beginning of each new switching cycle with both thresholds. If the signal exceeds the upper threshold, a fast recirculation demagnetization phase is performed. If the signal exceeds the lower threshold but not the upper threshold, a slow recirculation demagnetization phase is performed. If the signal is below the lower threshold, a magnetization phase is performed. The selection of the phase that is made is retained until the start of the following switching cycle.
According to yet another embodiment of the invention, particularly suited in the case of a constant frequency modulation, the signal representative of the current flowing in the load is compared at the start of each cycle with both thresholds. Based upon the comparison, the following functions may be performed.
If the signal exceeds the upper threshold, a fast recirculation demagnetization phase is performed. If during such a demagnetization phase the signal drops below the lower threshold, a slow recirculation demagnetization phase is performed that lasts until the following switching period begins.
If the signal exceeds the lower threshold but not the upper threshold, a slow recirculation demagnetization phase is performed. If during such a phase the signal exceeds the upper threshold (because of an abrupt increase of the current), a fast recirculation demagnetization phase is performed that lasts until the following switching period begins.
If the signal is below the lower comparison threshold, a magnetization phase is performed. Preferably, though not necessarily, the upper threshold and the lower threshold may be established to be substantially symmetric with respect to the reference value that represents the desired current value through the load.